Lithium polymer batteries are typically built as large format batteries of 20 kWh or more for use in electric vehicle, in stationary applications for back-up to ensure continuity to applications that cannot afford a grid power outage such as telecommunication stations, data centers, etc., or to provide alternate power source for peak shaving purposes in industrial or residential buildings.
Lithium polymer batteries consist of a plurality of electrochemical cells connected in series enclosed in a rigid casing which protect the electrochemical cells. Each electrochemical cell includes a plurality of elementary cell laminates connected in parallel. Each laminate includes an anode or negative electrode, a cathode or positive electrode, and a solid electrolyte comprising a polymer and a lithium salt separating the positive electrode from the negative electrode and providing ionic conductivity between the electrodes. The negative electrode may be a lithium or lithium alloy metal sheet or an active material capable of insertion and de-insertion of lithium ions such as carbon or Li4Ti5O12 in a polymer binder while the positive electrode consists of electrochemically active material particles such as LiFePO4, LiMnO2, LiMn2O4, etc., an electronically conductive additive and a solid polymer electrolyte which acts as a binder as well as provides the required ionic path between the electrochemically active material particles of the positive electrode and the solid electrolyte separator.
Contrary to lithium ion batteries which use a liquid electrolyte, lithium polymer batteries uses a solid electrolyte rendering this technology extremely safe. However, to obtain optimal ionic conductivity and therefore optimal performance, the electrochemical cells must be heated to temperatures of 60° C. to 80° C. Lithium polymer batteries therefore include a heating system to maintain the battery at a nominal temperature of 40° C. and to rapidly raise the temperature of the electrochemical cells to between 60° C. and 80° C. at the beginning of their discharge mode to obtain optimal performance from the battery. Once the optimal temperature is reached, the discharge operation generates sufficient heat to maintain the battery at its optimal temperature.
In operation, the excess heat generated by the plurality of electrochemical cells making up the battery is dissipated through the walls of the battery casing. The battery casing is preferably made of a rigid and heat conductive material such as aluminum or alloy thereof that efficiently conducts the excess heat outside the battery casing and there may be a cooling system outside the battery casing to accelerate heat dissipation when require.
In normal discharge operation, it was found that in a stack of electrochemical cells encased in a battery casing as described above, the electrochemical cells located adjacent to the walls of the battery casing were the first to reach their end of discharge voltage thereby marginally lowering the overall discharge capacity of the battery. This phenomena was attributed to the fact that these particular electrochemical cells were operating at slightly lower operating temperatures because they were losing heat more rapidly through the walls of the battery casing than the other electrochemical cells located farther away from the walls of the battery casing
Thus, there is a need for a battery casing and electrochemical cells configurations adapted to compensate for heat loss through heat sinks of the battery casing.